Weakness of the ankle or foot can be a significant factor in mobility impairments such as an abnormal gait pattern. One example of this phenomenon is observed in a condition known as drop-foot. A frequent complication associated with drop-foot is an inability to control the falling of the foot after heel strike, so that it slaps the ground at every step, which is referred to as foot-slap. Another complication associated with drop-foot is an inability for toes to clear the ground during swing phase of gait, which causes individuals with drop-foot to drag their toes on the ground throughout swing. Causes of drop-foot can include severed nerves, stroke, cerebral palsy and multiple sclerosis.
Weakness of ankle or foot muscles can also alter the gait pattern of the elderly. Age related changes in stereotypic movements such as walking patterns have been reported as early as age 60. Some of the age-associated changes in gait include a shortened step length, prolonged stance phase and double support times, and decreased ankle push-off power. Efforts to understand impaired mobility of the elderly have been focused on issues related to body support and forward propulsion.
Changes in patterns of mobility as a consequence of neurological or age related disorders can result in unsafe and energy consuming movement. In the case of drop foot, for example, accidental falls can occur during walking when the person catches their toes on the ground. A lack of balance control is also a major potential cause of falls involving a significant risk of injury. To provide another example, patients often compensate for drop-foot by excessively raising their hip or knee to ensure clearance of the foot off the ground, which is referred to as a steppage gait and which resembles the gait of a high-stepping horse. A steppage gait pattern expends more energy than normal, causing fatigue and making it more difficult and dangerous to walk.
An ankle-foot orthosis (AFO) is an orthopedic device that can provide support, stability, or replacement of lost function to the ankle. In North America, there are approximately 100,000 ankle-foot orthoses prescribed for children and adults with neuromuscular disorders such as spina bifida and cerebral palsy. Ankle orthotics can be useful after an acute ankle injury, for rehabilitation, to prevent ankle re-injury, for fall prevention in the elderly, and for chronically unstable ankles.
In a recently developed ankle-foot orthosis, the impedance of the orthotic joint was modulated throughout the walking cycle to treat drop foot gait. See Joaquin A. Blaya, Force-controllable ankle foot orthosis (AFO) to assist drop foot gait, Master's thesis, Massachusetts Institute of Technology, 2003, which is incorporated by reference herein in its entirety. It was found that actively adjusting joint impedance reduces the occurrence of slap foot, allows greater powered plantar flexion, and provides for greater biological realism in swing phase ankle dynamics.
Control of a human-exoskeleton system such as an ankle-foot orthosis presents significant challenges due to the complexity of the central nervous system (CNS) control and the interface between voluntary control and external artificial control. When humans interact with an external force field such as an exoskeleton, the central nervous system needs to learn an internal model of the force field and interaction with the force field. See R. Shadmehr, T. Brashers-Krug, and F. Mussa-Ivaldi, Interference in learning internal models of inverse dynamics in humans, in G. Tesauro, D. S. Touretsky, and T. K. Leen, eds., Advances in Neural Information Processing Systems, chapter 7, pages 1117-1224, MIT Press, 1995, which is incorporated by reference herein in its entirety. Therefore, a major challenge in the design and use of ankle-foot orthoses for daily activities relates to the coupled control of a human-exoskeleton system.
Another challenge to controlling an ankle-foot orthoses is that an expected trajectory is not available to the controller because the intended human motion cannot be predicted in advance by an exoskeleton controller. Human motion generally takes place in a dynamic environment and forces that will act on a body are also unpredictable. The inability to predict the intended motion in addition to interaction with uncertain dynamic environments creates a need for online or real-time control of an ankle-foot orthoses.
Conventional techniques for human-exoskeleton control tend to rely on unreliable calculation of first and second order time derivatives of noisy generalized coordinates. Conventional exoskeleton controllers are also susceptible to uncertainties in measurement of body parameters such as body segment mass, center of mass, and length. For example, conventional inverse dynamics control requires precise dynamic models because it is sensitive to parametric uncertainties. Further, coupled control of a human-exoskeleton system may lead to mechanical and metabolic inefficiencies if the assist controller is not properly designed.
For an ankle-foot orthosis, there is a need for practical and effective control strategies that can address mobility impairments such as abnormal gait patterns without the errors caused by calculation of higher order derivatives of noisy kinematic data. There is a need for ankle-foot orthosis controllers that are compatible with complex voluntary control performed by the central nervous system and that are capable of energy efficient, real-time control.